Such a blade is known from U.S. Pat. No. 3,874,816A, which discloses a rigid blade root end member fitted on the downstream side of the blade. This end member serves mainly to reinforce the blade structures, which in the case of this US patent specification is made of flexible material such as cloth and clamped on the connection part.
In the prior art, where the blade is made of a rigid, non-deformable material, and where the wind-energy-absorbing profile is expensive to produce and is of an elongated design, it is opted to fit a connection part between the wind-energy-absorbing part and the shaft/hub of the wind turbine. On the one hand, the connection part is cheap to produce and, on the other hand, it is designed in such a way that simple connection is possible, large torques being effectively transmitted. Since the torque of the aerodynamic forces increases with decreasing distance from the shaft, and since the surface area passed through by a blade pan decreases—and consequently so does the share in the energy output—with decreasing distance from the shaft, the blade cross section in the direction from tip to connection end acquires an optimization which runs from generally aerodynamic to generally structural. In particular, this connection part will comprise a tube. Since the connection part is situated near the centre of rotation of the wind turbine, the loss that occurs through the cross section not being optimized for the wind flow is less than the costs necessary for making a profile that is optimized for the wind flow, such as that fitted on the other side of the connection part. If the connection part is, for example, a tube, the costs of that tube per metre are many times lower than the costs of the wind-energy-absorbing, wing-like profile fixed on it. In addition, it is simple to connect a tube in a particularly sturdy manner to the remaining part of the structures, by means of, for example, a flange. The term connection part in the description and claims should be understood as meaning that part of a blade that is optimized for the structures, and not so much optimized for the absorption of wind energy. Such a part is characterized by a low natural speed, and it absorbs little in the way of wind forces. In general, it can be said that it does not comprise the part from the free end onwards with increasing chord, but it does comprise the part beyond the maximum chord, in other words the part with decreasing or invariable chord. The connection part can be connected with a separate flange to the aerodynamic wind-energy-absorbing profile.
Such blades are used on a large scale in the prior art.